The role of Preimplantation Factor (PIF) on leukocyte recruitment in vivo

Throughout pregnancy, immune cells infiltrate and colonize the placenta to ensure fetal development and successful birth. Thereby, they regulate tissue remodeling and protect the unborn from invading pathogens. At the same time, immune cells within the placenta require tight regulation in order to prevent recognition of the embryo as a ’semi-allograft’. Extra-embryonic tissue actively modulates immune cell functions by expressing growth factors and cytokines. Preimplanation factor, a 15 amino acid small peptide, is produced by trophoblast cells and continuously secreted into maternal circulation. It has been shown to interfere with immune cell functions in autoimmune disease models, but underlying molecular mechanisms remain unclear. This work investigated the function of PIF in acute inflammatory scenarios, reflecting its role within maternal serum. Analysis of leukocyte recruitment in postcapillary venules of TNF-α stimulated cremaster muscles in the mouse revealed that (i) leukocyte rolling, (ii) leukocyte adhesion and (iii) neutrophil extravasation is impaired in the presence of PIF. With the help of several ex vivo and in vitro assays, reduced leukocyte rolling could be linked to effects of PIF on the endothelial compartment. Impaired leukocyte adhesion and reduced extravasation in turn could be attributed to a direct effect of PIF on neutrophils. PIF inhibits K+ efflux via the voltage gated potassium channel KV1.3 on neutrophils, thereby reducing sustained calcium influx into the cells. Decreased intracellular Ca2+ concentrations impair post-arrest modification steps, namely neutrophil spreading and adhesion-strengthening, resulting in increased susceptibility to physiological shear forces and in reduced adhesion and extravasation. Taken together, this work demonstrates that PIF modulates neutrophil function during immune responses, offering therapeutic potential beyond pregnancy to protect patients from exuberant inflammation and excess neutrophil recruitment